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mm: page_counter: mitigate consequences of a page_counter underflow
When the unsigned page_counter underflows, even just by a few pages, a cgroup will not be able to run anything afterwards and trigger the OOM killer in a loop. Underflows shouldn't happen, but when they do in practice, we may just be off by a small amount that doesn't interfere with the normal operation - consequences don't need to be that dire. Reset the page_counter to 0 upon underflow. We'll issue a warning that the accounting will be off and then try to keep limping along. [ We used to do this with the original res_counter, where it was a more straight-forward correction inside the spinlock section. I didn't carry it forward into the lockless page counters for simplicity, but it turns out this is quite useful in practice. ] Link: https://lkml.kernel.org/r/20210408143155.2679744-1-hannes@cmpxchg.org Signed-off-by: Johannes Weiner <hannes@cmpxchg.org> Acked-by: Michal Hocko <mhocko@suse.com> Acked-by: Chris Down <chris@chrisdown.name> Reviewed-by: Shakeel Butt <shakeelb@google.com> Cc: Hugh Dickins <hughd@google.com> Cc: Roman Gushchin <guro@fb.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -52,9 +52,13 @@ void page_counter_cancel(struct page_counter *counter, unsigned long nr_pages)
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long new;
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new = atomic_long_sub_return(nr_pages, &counter->usage);
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propagate_protected_usage(counter, new);
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/* More uncharges than charges? */
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WARN_ON_ONCE(new < 0);
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if (WARN_ONCE(new < 0, "page_counter underflow: %ld nr_pages=%lu\n",
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new, nr_pages)) {
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new = 0;
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atomic_long_set(&counter->usage, new);
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}
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propagate_protected_usage(counter, new);
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}
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/**
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